Bone anchors may be used in orthopedic surgery to fix bone during the healing or fusion process. In spinal surgery, bone anchors may be used with spinal fixation elements, such as spinal rods, to stabilize multiple vertebrae either rigidly, in which no relative motion between the vertebrae is desired, and dynamically, in which limited, controlled motion between the vertebrae is desired. A closure mechanism is typically used to secure the spinal fixation element between two spaced apart arms of the receiver member of the bone anchor. In certain bone anchor assemblies, the spinal fixation element is also positioned between the spaced apart arms of a compression member positioned within the receiver member. Tightening of the closure mechanism can cause deformation of the components of the bone anchor assembly including, for example, the receiver member or the compression member. Such deformation can cause the arms of the receiver mechanism or the arms of the compression member to separate or splay, which can result in the closure mechanism loosening over time and, in the worst case, the spinal fixation element separating from the bone anchor assembly.
Accordingly, there is a need for improved bone anchor assemblies in which deformation of the components of the assembly is minimized during tightening.